Study On The Stress Fracture-Seepage-Law Of Coal In The Protected Layer In The Mining Of The Upper Protective Layer

Improving the permeability of coal seams is of great significance for the efficient extraction of coalbed methane.Protective layer mining is widely used as a common means of increasing permeability.However,the mining of coal seams with multiple coal seams will cause the coal body to be repeatedly affected by loading and unloading,and the permeability level is difficult to predict.It increases the difficulty of mining coalbed methane.In order to study the seepage characteristics of the protected seam under the repeated mining stress of the coal seam group,by summarizing and analyzing the stress change path of the existing protective seam mining,combined with the stress distribution of the intact,fractured and broken coal seams during the mining of the protective seam on the coal seam group in Huaibei mining area According to the law,the seepage experiment of repeated loading and unloading of coal was carried out.According to the stress distribution law of protective layer mining on the coal seam group,the stress path of complete and fractured coal seams is divided into three stages:stress compression(loading)stage,pressure relief expansion(unloading)stage and stress recovery(recovery)stage;The stress path is divided into two stages: unloading expansion(unloading)stage and stress recovery(recovery)stage.Substitute the complete coal samples with through-fractures(vertical,inclined)and broken(coal,rock)samples into the above two stress paths to carry out cyclic loading and unloading experiments,and observe the change law of permeability of the three coal bodies.Specifically,the following conclusions are drawn:(1)When the upper protective layer is mined,the protected layer will be subjected to cyclic loading and unloading.The state of the coal seam in front of the working face of the protective layer is mostly complete and with cracks,and the coal seam in the goaf is mostly in a broken state.The stress path of intact,fractured coal samples is divided into three stages: stress compression stage,pressure relief expansion stage and stress recovery stage.The stress paths of broken coal samples are: stress recovery stage and pressure relief expansion stage.(2)When the upper protective layer is mined,the protective layer in front of the working face will have the effect of cyclic loading and unloading of the protected layer.The cyclic loading and unloading has certain damage and damage to the intact and fractured coal samples,and with the increase of the number of cycles,The impact of the mining layer on the protected layer is more severe.The pressure relief expansion stage is the main stage that affects the permeability change of intact and fractured coal samples.The effect of cyclic loading and unloading on fractured coal samples is greater than that on intact coal samples.(3)During the mining of the upper protective layer,the broken coal seam mainly has three change forms: extrusion deformation,structural adjustment and particle crushing.The permeability of the broken coal seam shows a downward trend as a whole.After one mining,the permeability of the broken coal seam decreases significantly.The influence of the loading stage in the stress path of the protected layer on the broken coal seam is greater than that of the unloading stage.(4)After the intact coal sample is covered by the stress path of the protective layer,the change trend of permeability under different pore pressure is basically the same;After the fractured coal sample passes through the stress path of the protected layer,the variation range of permeability under different pore pressure decreases gradually with the increase of the number of cycles;After the broken coal sample passes through the stress path of the protected layer,the permeability curve changes under different pore pressure are basically the same,and the overall strength of the broken mixed sample is greater than that of the broken coal sample.(5)When the protected layer is a complete coal seam,the extraction position of the protected layer is close to the position behind the working face.With the approach of the protective layer,the extraction pressure increases gradually.When the protected layer is a fractured coal seam,the extraction position is consistent with the complete coal seam.When the protective layer is close to the protected layer,the extraction pressure is basically unchanged.When the protected layer is a broken coal seam,the drainage position is located behind the complete and fractured coal seam.The gas in the early stage of the broken coal seam will be actively desorbed with the mining action,but with the increase of the number of protective layers,the drainage pressure needs to increase gradually.There are 53 figures,11 tables and 114 references in this thesis.